<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    body {
      width: 100vw;
      height: 100vh;
      background-color: black;
    }

    div {
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
      width: 400px;
      height: 200px;
      border-radius: 10px;
      background-color: white;
      font-size: 50px;
      line-height: 200px;
      text-align: center;

      /*
        -webkit-box-reflect: 方向 偏移量 蒙版
         ● 方向：above below left right
       */
      -webkit-box-reflect: below 0 linear-gradient(rgba(255, 255, 255, 0.3), rgba(255, 255, 255, 0.7));
    }
  </style>
</head>

<body>
  <div>Hello world!</div>
</body>

</html>